Not applicable.
This invention relates to a method of producing dicarboxylic acids suitable for polymer or polyamide synthesis. The carboxylic acids comprise saturated dicarboxylic acids with a chain length of C6 to C21 produced by fatty acid cleavage of unsaturated fatty acids by means of oxidative ozonolysis and subsequent separation and purification of the dicarboxylic acids. Furthermore, the dicarboxlyic acids comprise diamidic saturated dicarboxylic acids in the form of bis-dicarboxylic acid diamides of saturated dicarboxylic acids with a chain length of C6 to C21 produced by cleavage of bis-fatty acids diamides of unsaturated fatty acids by oxidative ozonolysis and subsequent separation and purification of the diamidic dicarboxylic acids.
The fatty acid cleavage or splitting of unsaturated ative fatty acids to mono- and dicarboxylic acids which is used to produce dicarboxylic acids is known in general. The best known example of this in the relevant chemistry references is oxidative ozonolysis of oleic acid to pelargonic acid and azelaic acid. In oxidative ozonolysis, the unsaturated fatty acids are reacted with ozone to form ozonides, which are reacted directly, i.e., without isolating them, by oxidation to form the carboxylic acids. In this way, for example, oleic acid (C18:1-carboxylic acid) can be cleaved to form pelargonic acid (C9-monocarboxylic acid) and azelaic acid (C9-dicarboxylic acid).
In addition, it is also known that in the case of unsaturated native fatty acids, the two-step reaction described below is carried out to advantage in pelargonic acid as the solvent and in the presence of water. The weight ratios of fatty acid to solvent is approx. 1:1. The amount of water added depends on how the process heat is dissipated in the respective manner of managing the process. It is advantageous to use pelargonic acid because it is formed in the process anyway. Adding water prevents the formation of unwanted by-products in ozonolysis, but water also serves as a medium to absorb and dissipate the reaction heat in oxidation of the double bond of the fatty acid to the ozonide ring and oxidative cleavage of the latter to form carboxylic acids.
The reaction of monounsaturated fatty acids such as oleic acid with ozone as well as purification of the derivative products formed by oxidative ozonolysis, such as pelargonic acid and azelaic acid from oleic acid, have proven to be difficult. The main reasons include:
1. The reaction components used are not pure compounds such as pure oleic acid, but instead they are concentrates in which the component to be reacted is present in varying concentrations. In production technology, oleic acid is used in the form of a 70% to 80% concentrate (percent by weight of the type of monounsaturated oleic acid desired for the cleavage reaction, based on the total amount) containing saturated fatty acids as well as other monounsaturated fatty acids and polyunsaturated fatty acids.
2. Since oxidative ozonolysis breaks the starting compound down into two fragments, which are approximately equal in size in the case of oleic acid, the ratio of impurities to the desired substance changes to the detriment of the derivative compounds. For example, if 70% oleic acid concentrate contains 10 wt % saturated fatty acids and thus if the weight ratio of oleic acid to saturated fatty acids is 7:1, then after the reaction, the weight ratio of azelaic acid and pelargonic acid to saturated fatty acids is 3.5:1. This makes it difficult to isolate the pure derivatives in high yield.
3. As an extremely reactive oxidizing agent, ozone can also attack components of the reaction solution other than the desired reaction component(s). This is also true of the main component of the reaction solution which has already been converted to an ozonide according to this invention.
4. The reaction of ozone with double bonds involves a number of extremely unstable and reactive intermediates (see Organikum [Organic Chemistry], VEB Deutscher Verlag der Wissenschaften [People""s Science Publishing House], eighth edition, 1968, page 252). These reactive intermediates can lead to unwanted by-products such as per acids, per esters and per ethers.
5. Polyunsaturated fatty acids react in oxidative ozonolysis to form short-chain mono- and dicarboxylic acids. This makes it difficult to isolate pure derivatives.
6. Polyunsaturated fatty acids contain active methylene groups which lead to unwanted side reactions, such as a shifting of the double bonds, cross-linking with other fatty acids and polymerization, especially after oxidative attack.
In industrial production technology, oxidative ozonolysis of unsaturated fatty acids is used mainly with oleic acid concentrate from tallow, with pelargonic acid serving as the reaction solvent, and mainly azelaic acid being formed as the dicarboxylic acid and pelargonic acid as the monocarboxylic acid. The unwanted by-products and derivatives can be minimized through a suitable choice of reaction conditions. Thus, oxidative ozonolysis of oleic acid concentrate is carried out in two steps, with the actual ozonolysis being performed at temperatures of less than 50xc2x0 C. in a first reactor by addition of ozone onto the double bond with rearrangement to the ozonide ring. To prevent unwanted side reactions, the reaction heat is dissipated by means of intense heat exchange. Therefore, the reaction is carried out in the presence of water as the heat-absorbing and heat-dissipating medium. Then the oxidation of the ozonide ring with oxygen takes place at approximately 100xc2x0 C. in a second reactor. The organic phase and the aqueous phase are brought in intense contact with the gaseous oxygen. Here again, the reaction heat released is dissipated by intense heat exchange with water to prevent unwanted side reactions.
With a two-step process, a yield of greater than 90% of the theoretical amount of pelargonic acid and azelaic acid is obtained, based on the starting oleic acid. A two-step oxidative ozonolysis with such yields is described, for example, in the article by Dr. Martin Witthaus of Henkel KGaA xe2x80x9cOzonolysis of unsaturated fatty acidsxe2x80x9d from the publication series of the Foundation of the Chemical Industry, no. 26, Frankfurt am Main, 1986.
The resulting products, in particular the resulting dicarboxylic acid azelaic acid, are obtained in complicated purification operations according to the state of the art. The pelargonic acid formed in the reaction and also used as a solvent can be recovered by distillation, with short-chain monocarboxylic acids being distilled over as the first runnings. In the case of oxidative ozonolysis of oleic acid concentrate, the resulting dicarboxylic acids can be obtained from the residue of pelargonic acid distillation by high-temperature vacuum distillation and/or by elution with water. The azelaic acid thus obtained is technically pure, but it still contains residues of monocarboxylic acids and other dicarboxylic acids, especially those with a shorter chain length.
To produce azelaic acid for polymer synthesis, the azelaic acid obtained in technical-grade purity must be further purified. For fine purification of the products, the respective mono- and dicarboxylic acid mixtures can also be subjected to fractional distillation, preferably after conversion to the methyl ester. Azelaic acid purified by this complicated procedure and having a dicarboxylic acids content of  greater than 99 wt % is converted to the diammonium salts of the corresponding diamines and condensed for the purpose of polyamide synthesis. Complicated purification processes of the type described here for azelaic acid can be found in U.S. Pat. No. 3,402,108, for example.
In oxidative ozonolysis of erucaic acid (C22:1), forming pelargonic acid and brassylic acid, brassylic acid (C13-dicarboxylic acid) cannot be obtained directly by distillation or eluted with water at a justifiable expense. The remaining traditional purification operations here include a time-consuming and tedious recrystallization from suitable solvents such as acetone, acetonitrile and ethanol/water and distillation of the esters (preferably the methyl ester) of the acid thus obtained.
Accordingly, it is customary in the related art to produce pure brassylic acid by the sequence of processes including 1. esterification of crude brassylic acid to methyl ester, 2. fractional vacuum distillation of the methyl ester and 3. saponification of the pure methyl ester. Since the methyl ester is used in many applications, synthesis of the pure acid can be omitted in these cases. Even the first purification step in this process chain involving synthesis of the ester and fractional vacuum distillation is associated with a great expense and a loss of yield.
The literature (see xe2x80x9cReactions of Azelaaldehydic Esters,xe2x80x9d by E. H. Pryde and J. C. Cowan, J. Am. Oil Chem. Soc., volume 46, 1969, pages 213 to 218) describes ozonolysis of dioleodiamides and bis-oleic acid diamides such as those obtained by the reaction of diamines, such as ethylenediamine, with oleic acid. This process has not been carried out on an industrial scale. Bis-oleic acid diamide can be converted by oxidative ozonolysis to the respective diamidic dicarboxylic acid diazelaindiamide or bis-azelaic acid diamide, with the NH groups attached to the preferably long-chain structure, as described in the literature citation immediately above, for example. This dicarboxylic acid diamide is an interesting polymer building block, especially for synthesis of polyamides. It should be possible to convert bis-erucaic acid diamide to bis-brassylic acid diamide, which is also an interesting polymer building block, by a similar procedure.
However, the use of the diamidic dicarboxylic acid, namely bis-azelaic acid diamide, thus produced as a building block for high molecular polymers presupposes a very high purity. Therefore, according to the literature reference cited immediately above, an attempt was made to purify the filtrate of the reaction end product of oxidative ozonolysis which had cooled overnight by mixing it thoroughly and repeatedly with ether. The reaction product which was filtered and left to stand overnight also contained formic acid as a reaction solvent. However, according to the findings of the inventors of the present patent application, the melting range given in this literature reference for the bis-azelaic acid diamides thus obtained does not characterize building blocks of adequate purity for this purpose. For a sufficiently finely purified product, the melting range would have to be higher than that given there.
However, inadequate purification has some serious consequences. If such dicarboxylic acid diamides are synthesized from an oleic acid concentrate, mixed diamides such as stearo-oleodiamide are always formed in addition to the symmetrical dioleodiamides and can cleaved in oxidative ozonolysis to form mono-reactive diamide compounds. The latter lead to chain break-off in polymerization and thus cause a significant decline in overall properties of the resulting polymers if not removed adequately.
The object of the present invention is to provide a method that will make it possible in the simplest and least expensive manner to produce the desired saturated dicarboxylic acids suitable for synthesis of polymers or polyamides. Such dicarboxylic acids include xe2x80x9cnon-diamidicxe2x80x9d dicarboxylic acid, such as those which can be obtained by oxidative ozonolysis of unsaturated fatty acids as well as xe2x80x9cdiamidicxe2x80x9d dicarboxylic acids in form of bis-dicarboxlic acid diamides such as those obtained by oxidative ozonolysis of bis-fatty acid diamides of unsaturated fatty acids. The structure of these bis-dicarboxylic acid diamides which are still to be regarded as dicarboxylic acids because they still comprise acid groups, is as follows:
Bis[dicarboxylic acid]diamide: 
for example Bis[azelaic acid]ethylene diamide: 
wherein the acid residue R1 is (CH2)7, and the amino residue R2 is CH2-CH2.
The principal structure can also be taken from the above cited document of Pryde and Cowan.
When dicarboxylic acids are mentioned below, these statements apply in general to both diamidic dicarboxylic acids and dicarboxylic acids that are non-diamidic.
The above object underlying the invention is solved by the method of the appended independent claims. Advantageous refinements of these methods are defined in the subordinate claims.